1. Field of the Invention
The present invention relates generally to devices for measuring the thickness of objects and, more particularly, to devices having eddy current sensors for measuring thickness.
2. Description of Related Art
Eddy current sensors are non-contact measurement devices used for measuring the thickness of conductive objects. Briefly, an eddy current sensor includes a sensor coil, which when driven by an AC current, generates an oscillating magnetic field that induces an eddy current in the surface of a nearby conductive object. The eddy current is dependent on the strength of the magnetic B-field created by the AC current and the impedance of the object, which is related to the thickness of the object and the resistivity of the object. The thickness of the object can be determined based on the known resistivity of the object and the measured eddy current or impedance.
In semiconductor manufacturing, one common use of eddy current sensors is for measuring the thickness of a conductive layer (such as, e.g., a copper layer) deposited on a wafer substrate. Eddy current sensors are used for determining the thickness of a conductive layer at various sampling locations on the wafer. In many cases, it is important to have a generally uniform conductive layer thickness to avoid problems in subsequent processing such as etching. It is accordingly important to be able to accurately determine the thickness of conductive layers so that corrective action can be taken, if needed, to obtain a desired thickness. Alternatively, the wafer can be scrapped to avoid the unnecessary expense of further processing.
Currently available eddy current sensor devices for measuring the thickness of conductive layers on wafers are generally very slow. These devices can also be very sensitive to inadvertent movement of the object relative to the eddy current sensors and, accordingly, often have complex and costly position control mechanisms in an attempt to provide a generally uniform distance between the sensor and the wafer.